1. Field of the Invention
This invention relates to transistor amplifier output circuits and, in particular to protective circuitry which ensures safe operation of the output transistors of a transistor amplifier which has a reactive load.
2. Description of the Prior Art
In general, the output stage of double power source, class B, single ended push-pull amplifiers (hereinafter class B SEPP amplifier) is as typically shown in FIG. 1 where an NPN output transistor 1 and a PNP output transistor 2 are connected in series with emitter resistances 5 and 6 respectively between two power sources +Vcc and -Vcc. A load 3 is connected between ground and a common output terminal 4 of the emitters of output transistors 1 and 2. ei is the input signal source.
If load 3 is a reactive, a phase difference develops between the current I flowing into transistor 1 and the load voltage across load 3. For example, current I flows into transistor 1 when the electric potential of common output terminal 4 oscillates up to power source -Vcc. The actuation locus of output transistor 1 when a reactive load is connected as load 3 is described by an elliptical locus with a portion omitted as in the case of the solid line A in FIG. 2 where the horizontal axis is the voltage V.sub.CE of the emitter-collector of the transistor and the vertical axis is the emitter current I. The actuation locus of output transistor 1 would be a straight line, such as the broken line B in FIG. 2, when pure resistance is connected as load 3. This relation is identical for output transistor 2.
When a reactive load is connected as load 3, there is a danger of destruction if transistors with large regions of safe operation are not used as output transistors 1 and 2. Moreover, the transistors will cease to circulate output current as the voltage approaches the amount of voltage which the transistors themselves can withstand.